1. Field of the Invention
The present invention relates to a transparent conductive film useful for e.g. a liquid crystal display (LCD) and a process for forming a transparent electrode.
2. Discussion of the Background
At present, a film made of a mixed oxide of In and Sn (hereinafter referred to as InSn.sub.x O.sub.y) is widely employed as an electrode for LCD. Particularly in a STN type color LCD, along with the progress for high resolution and a large image screen, the transparent electrode for driving liquid crystal is required to have a narrower line width and a longer shape.
Accordingly, a transparent conductive film having an extremely low resistance with a sheet resistance of not higher than 3.OMEGA./.quadrature. is required. To accomplish such a sheet resistance, it is necessary to increase the film thickness (at least 300 nm) of the transparent conductive film or to reduce the resistivity (not higher than 100 .mu..OMEGA..multidot.cm).
However, there is a limit -in increasing the film thickness, since 1) the film forming cost for the transparent conductive film increases, 2) difficulty in patterning the electrode increases, and 3) a step difference increases as between the presence and the absence of the transparent electrode, whereby there will be a problem that control of the alignment of liquid crystal molecules will be difficult.
On the other hand, a method of reducing the resistivity of the InSn.sub.x O.sub.y film itself has been studied, but a method for constantly producing an InSn.sub.x O.sub.y film having a low resistance of not higher than 100 .mu..OMEGA..multidot.cm, has not yet been established.
On the other hand, as a method for readily obtaining transparent conductive film having a low resistance of not higher than 100 .mu..OMEGA..multidot.cm, it is known to sandwich a Ag layer between InSn.sub.x O.sub.y layers to have a structure of InSn.sub.x O.sub.y /Ag/InSn.sub.x O.sub.y. Such a structure provides a low resistivity, but the durability is so poor that when left to stand in a room, white defects will form which appear to be peeling off the film. Further, also at the time of processing the electrode pattern by etching by means of an acidic aqueous solution, side etching tends to proceed, whereby peeling is observed at a pattern edge portion, and thus the processability is inadequate.
Accordingly, a substrate having a structure of InSn.sub.x O.sub.y /Ag/InSn.sub.x O.sub.y has not yet been practically used as a transparent conductive substrate for LCD, even though it has the advantage that low resistance can easily be attained.